In recent years, there has been an increased interest in thermal printers and, especially, in dye thermal transfer printers which allow printing of clear full-color images. A dye thermal transfer printer forms an image by placing a dye-containing layer of the ink sheet onto an image-receiving layer (hereinafter also referred to simply as “receiving layer”) comprising a dye-fixable resin on the receiving sheet, and then supplying heat from a thermal head or the like so as to transfer the dye at a predetermined location of the dye layer of the ink sheet to the receiving layer. The ink sheets comprise dye layers of three colors: yellow, magenta and cyan, or four colors, if black is also included. A full-color image is obtained by repeatedly transferring the dye of each of the colors in sequence to the receiving sheet. Because dye thermal transfer systems allow high-quality image recording, and are also suitable for digital printing from recently popular digital cameras, such systems are gradually replacing silver salt photography.
The receiving sheet is associated with a drawback of a poor image stability, because the dyes transferred to the receiving layer penetrate into the underlying layer over time, and are then diffused into the substrate (hereinafter also referred to as “bleeding”), whereby the image would lose clarity.
This drawback becomes particularly pronounced when it is attempted to improve the recorded image density or quality by forming an intermediate layer comprising hollow or foam particles on a base sheet so as to impart a cushioning property to the receiving sheet (for example, Japanese Unexamined Patent Publication (Kokai) No. 1-27996, Japanese Unexamined Patent Publication (Kokai) No. 63-87286).
The thermal insulation property, smoothness and cushioning property are essential features for efficient utilization of heat from the thermal head for printing, and they significantly affect the printed image quality and image density. More specifically, in the course of printing an image, the receiving sheet contacts with the thermal head via the ink sheet and is pressed from the opposite side by a rubber roll which is referred to as a “platen roll”. Under the pressure applied from the rubber roll, a receiving sheet with a good cushioning property will adhere completely to the ink sheet, with an absence of gaps, and allow uniform transfer of the ink for satisfactory image quality, but a receiving sheet with a poor cushioning property will adhere to the ink sheet with gaps between it and the contacting ink sheet, whereby the ink will be poorly transferred, due to the existence of gaps, resulting in a non-uniform image. Thus, the cushion property is one of the most important qualities of a receiving sheet. Japanese Unexamined Patent Publication (Kokai) No. 9-99651 discloses the preferred sizes for hollow particles in the intermediate layer (foam layer), for the purpose of achieving an enhanced printing quality.
A receiving sheet fabricated by providing an intermediate layer containing hollow or foam particles has a drawback of a significantly poor image stability, because the dyes transferred to the receiving layer penetrate into the underlying layer over time and, then, tend to be diffused into the substrate (bleeding), whereby the image would lose clarity. Thus, a layer with high barrier properties (a barrier layer) is essential in order to prevent bleeding particularly in receiving sheets having an intermediate layer comprising hollow or foam particles.
Japanese Unexamined Patent Publication (Kokai) No. 6-227159 proposes a method wherein a layer containing a lamellar inorganic pigment with an aspect ratio of 5-90 is provided on a hollow particle containing primer coating layer (intermediate layer), for the purpose of preventing penetration of the receiving layer coating components or the solvent used in the receiving layer coating composition. However, a layer containing such a lamellar inorganic pigment having an aspect ratio in the aforementioned range is not sufficient to prevent penetration of the image-forming dye into the intermediate layer or substrate, and thus exhibits virtually no bleed-preventing effects. One of the reasons for this is presumably that, unlike the penetration of the receiving layer coating components or the solvent used in the receiving layer coating composition, with respect to a dye used in sublimation thermal transfer, penetration of the dye occurs at a molecular level. Bleeding may be prevented by increasing the coverage of the barrier layer. However, excessive increase of the coverage of the barrier layer will reduce the thermal insulating effect of the intermediate layer, whereby reduces the printing density, and thus, results in unclear images. With ongoing substitution for silver salt photography in recent years, a demand exists for receiving sheets with higher image quality and a superior image stability, goals which require better techniques.